M. W. Beetz on the Colour of Water. 223 



A hollow prism made of glass plates is so brought under 

 water that the horizontal light from the surface is totally re- 

 flected from the hypothenuse. Instead of this apparatus, 

 Poggendorff* proposed a glass mirror inclined at 45° to the 

 horizon. I happened to have occasion to make a corresponding 

 experiment ; I wanted to fill a tinplate tube, which was closed 

 at the ends with glass plates and had a hole in the side, by 

 placing it in a very inclined position quite under the sea-level. 

 When the upper glass plate had the right inclination, in the 

 Tegernsee it reflected in sunny weather an emerald-green light 

 more intense than I have obtained in any other way ; in the 

 Achensee, however, a blue light, as if it had passed through 

 concentrated solution of sulphate of copper. Hence Arago's 

 proposal is appropriate ; and if he had had an opportunity of 

 carrying it out, he would certainly have given up the notion 

 that water shows different colours in reflected and in trans- 

 mitted light. 



The colour of water alters naturally, when solid particles are 

 suspended in it. By mixing such bodies which, like the above- 

 mentioned constituents of the ground, reflect red light in pre- 

 ference when they are moistened, it may yet appear red; by 

 greater masses of whitish sand which have become heaped up in 

 the lakes during a continuous storm, or which the rivers have 

 worn down from their beds, the water appears clearer than 

 otherwise. Simony f observes that the Wolfang and Attersee 

 appear in winter, when they are clearest, of a dark green, but 

 in summer bluish-green or cerulean blue, and he considers this 

 colour as occasioned more especially by the marl and grey 

 sandstone predominant in the debris. 



In the previous considerations, the influence of the colour of 

 the sky and of the surrounding neighbourhood has been dis- 

 regarded. Yet there are many who seek the cause of the colour 

 of water in these circumstances. But these secondary influences 

 must be taken into account along with the chief cause. When 

 the surface of the lake is quite clear, it acts as a mirror. The 

 special phenomena of colour are the more concealed, the more 

 regular reflected light reaches the eye from the place in question ; 

 they appear purest where no or but little light is regularly 

 reflected, for example, against a dark rocky background. But if 

 the sea is in motion, the regular reflexion always diminishes, 

 and the aspect of the surface is changed by the occurrence of 

 waves in a very complicated manner, depending on the formation 

 of the bank, the direction and intensity of the wind, and similar 



* Poggendorff' s Annalen, vol. xlv. p. 474. 

 t Wiener Sitzungsbericht, iv. 542. 



